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PROKARTOTES
Prokaryotes thrive almost everywhere, including places too
acidic, salty, cold, or hot for most other organisms
Most prokaryotes are microscopic, but what they lack in size
they make up for in numbers.
There are more in a handful of fertile soil than the number of
people who have ever lived
Prokaryotes are divided into two domains:
BACTERIA and ARCHEA
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Structural and functional adaptations
contribute to prokaryotic success
• Earth’s first organisms were likely prokaryotes
• Most prokaryotes are unicellular, although
some species form colonies
• Most prokaryotic cells are 0.5–5 µm, much
smaller than the 10–100 µm of many eukaryotic
cells
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Basic Structure of a Prokaryotic cell
Prokaryotic cells have a variety of shapes
The three most common shapes are spheres (cocci), rods
(bacilli), and spirals
1 m
1 m
3 m
Figure 27.2
(a) Spherical
(b) Rod-shaped
(c) Spiral
Cell-Surface Structures
• An important feature of nearly all prokaryotic
cells is their cell wall, which maintains cell shape,
protects the cell, and prevents it from bursting in
a hypotonic environment
• Bacterial cell walls contain peptidoglycan, a
network of sugar polymers cross-linked by
polypeptides
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• Archaea contain polysaccharides and proteins
but lack peptidoglycan
• Scientists use the Gram stain to classify bacteria
by cell wall composition
• Gram-positive bacteria have simpler walls with
a large amount of peptidoglycan
• Gram-negative bacteria have less peptidoglycan
and an outer membrane that can be toxic
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Figure 27.3
(a) Gram-positive bacteria: peptidoglycan traps crystal violet.
Gram-positive
bacteria
(b) Gram-negative bacteria: crystal violet is easily rinsed
away, revealing red dye.
Gram-negative
bacteria
Carbohydrate portion
of lipopolysaccharide
Cell
wall
Peptidoglycan
layer
Cell
wall
Plasma
membrane
10 m
Outer
membrane
Peptidoglycan
layer
Plasma membrane
Figure 27.3c
Gram-positive
bacteria
Gram-negative
bacteria
10 m
• Many antibiotics target peptidoglycan and
damage bacterial cell walls
• Gram-negative bacteria are more likely to be
antibiotic resistant ie: Escherichia coli, Salmonella,
Helicobacter, Stenotrophomonas,
• Gonorrhoeae, meningitis
• Gram-positive bacteria include
– Actinomycetes, which decompose soil
– Bacillus anthracis, the cause of anthrax
– Clostridium botulinum, the cause of botulism
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Figure 27.4
Bacterial
cell wall
Bacterial
capsule
Tonsil
cell
200 nm
• Some prokaryotes have fimbriae, which allow
them to stick to their substrate or other
individuals in a colony
• Pili (or sex pili) are longer than fimbriae and
allow prokaryotes to exchange DNA
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Motility
• In a heterogeneous environment, many bacteria
exhibit taxis, the ability to move toward or away
from a stimulus
• Chemotaxis is the movement toward or away
from a chemical stimulus
• Most motile bacteria propel themselves by
flagella scattered about the surface or
concentrated at one or both ends
• Flagella of bacteria, archaea, and eukaryotes are
composed of different proteins and likely evolved
independently
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• The prokaryotic genome has less DNA than the
eukaryotic genome
• Most of the genome consists of a circular
chromosome
• The chromosome is not surrounded by a
membrane; it is located in the nucleoid region
• Some species of bacteria also have smaller rings
of DNA called plasmids
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Figure 27.8
Chromosome
Plasmids
1 m
Reproduction and Adaptation
• Prokaryotes reproduce quickly by binary fission
and can divide every 1–3 hours
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• Many prokaryotes form metabolically inactive
endospores, which can remain viable in harsh
conditions for centuries
Coat
Endospore
0.3 m
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Rapid Reproduction and Mutation
• Prokaryotes reproduce by binary fission, and
offspring cells are generally identical
• Mutation rates during binary fission are low, but
because of rapid reproduction, mutations can
accumulate rapidly in a population
• High diversity from mutations allows for rapid
evolution
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Conjugation and Plasmids
• Conjugation is the process where genetic
material is transferred between prokaryotic cells
• In bacteria, the DNA transfer is one way
• A donor cell attaches to a recipient by a pilus,
pulls it closer, and transfers DNA
• A piece of DNA called the F factor is required
for the production of pili
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Figure 27.12
1 m
Sex pilus
R Plasmids and Antibiotic Resistance
• R plasmids carry genes for antibiotic resistance
• Antibiotics kill sensitive bacteria, but not bacteria
with specific R plasmids
• Through natural selection, the fraction of
bacteria with genes for resistance increases in a
population exposed to antibiotics
• Antibiotic-resistant strains of bacteria are
becoming more common
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Nutritional needs and Metabolism
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Table 27.2
• Archaea live in extreme environments and are
called extremophiles
• Extreme halophiles live in highly saline
environments
• Extreme thermophiles thrive in very hot
environments
• Methanogens live in swamps and marshes and
produce methane as a waste product
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Figure 27.16
Subgroup: Gamma Proteobacteria
• Examples include sulfur bacteria such as
Chromatium and pathogens such as Legionella,
Salmonella, and Vibrio cholerae
• Escherichia coli resides in the intestines of many
mammals and is not normally pathogenic
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Prokaryotes play crucial roles in the
biosphere
• Prokaryotes are so important that if they were to
disappear the prospects for any other life
surviving would be dim
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Chemical Recycling
• Prokaryotes play a major role in the recycling of
chemical elements between the living and
nonliving components of ecosystems
• Chemoheterotrophic prokaryotes function as
decomposers, breaking down dead organisms
and waste products
• Prokaryotes can sometimes increase the
availability of nitrogen, phosphorus, and
potassium for plant growth
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